This invention relates to a yarn composed of ultrafine fibers of synthetic thermoplastic material. In one aspect, it relates to a meltblowing process and apparatus for manufacturing meltblown yarn. In another aspect, the invention relates to a fabric made from meltblown yarn. In a further aspect, the invention relates to a filter made from melt-blown yarn.
Meltblown fabrics manufactured from a synthetic thermoplastics have long been used in a variety of applications including filters, batting, fabrics for oil cleanup, absorbents such as those used in diapers and feminine hygiene absorbents, thermal insulation, and apparel and drapery for medical uses.
Meltblown materials fall in the general class of textiles referred to as nonwovens owing to the fact they comprise randomly oriented fibers made by entangling the fibers through mechanical means. The fiber entanglement, with or without some interfiber fusion, imparts integrity and strength to the fabric. The nonwoven fabric may be converted to a variety of end use products as mentioned above.
While it is true that meltblown material may be made as a roving, as described in U.S. Pat. No. 3,684,415, the apparatus for manufacturing the roving according to this process, is expensive, complicated and unreliable. The apparatus and process described in U.S. Pat. No. 3,684,415, have received very little, if any, commercial application.
Recently, efforts to make meltblown rods suitable for cigarettes filters resulted in several patents. See, for example, U.S. Pat. Nos. 4,961,415, 5,053,066, 5,509,430 and 5,531,235. Cigarette filter rods, however, are compact and substantially inflexible, making them totally unsatisfactory as yarns.
Conventional yarns are manufactured by twisting aligned monofilament threads of natural or synthetic fibers such as cottons, wool, nylon, polyesters and polyolefins. The filaments in the threads have a relatively large diameter (20 to 30 micron range) compared to melt-blown fibers (less than 10 microns). Because the threads are aligned during the twisting step of the process, the yarn does not possess texture or bulk (i.e. low bulk density). Conventional yarns are therefore further processed to reduce the bulk density and impart bulk to the yarn.
In addition to the traditional uses of yarn, as in fabric manufacturing, conventional yarns are now being used in a wide range of filtration application. One popular filter using yarns or threads is the wound cartridge filter. Typical material used in these yarns include polypropylene, fibrillated polypropylene, polyethylene cotton, rayon, polyester, nylon, and heat treated glass fibers.
Wound cartridge filters are made by winding the yarn on a core. This produces a depth filter with diamond shaped tunnels that get progressively smaller from the outer diameter to the core. Finer particles are progressively trapped as fluid travels to the center of the filter, allowing a much greater retention capacity than that with straight surface media of the same dimensions and porosity.
The choice of winding material for a particular wound cartridge, is dependent on several factors including chemical resistance and heat resistance requirements, FDA approval requirements, non-leaching requirements, as well as nominal and absolute particle retention requirements. The relatively large size (20-50 microns) of the standard fibers used to produce traditional yarns, limits the porosity of the yarns and hence limits the size of the particles that can be retained from the liquid or air wound filter.
Meltblown webs of polypropylene have also been used in cartridge fibers. The microsized fibers in meltblown webs provide high surface area, an important feature of filters. Cartridge filters that employ meltblown webs are disclosed in U.S. Pat. Nos. 5,340,479 and 5, 409,642. Although meltblown webs have been used in cartridge filters, meltblown yarns have not. The industry recognized the importance of the meltblown microsized fibers (and attendant increased surface area of the filter media), but could not implement this feature in wound cartridges since yarns having microsized fibers (0.5 to 10 microns) were not available prior to the present invention.